Field of the Invention
The present invention relates to an image forming apparatus including a cassette on which sheets are to be placed.
Description of the Related Art
Hitherto, a sheet feeding device is arranged in an image forming apparatus such as a copying machine or a printer. In order to increase convenience when image formation is continuously performed on a large number of sheets, a large number of predetermined-size sheets are placed on a cassette (deck) of the sheet feeding device, and the sheets are fed by a separating/feeding mechanism to an image forming portion one by one. The sheet feeding device is required to have as large a sheet capacity as possible in order to reduce the number of times that a user supplies sheets. However, it is necessary to stack the sheets vertically in order to feed the sheets by the separating/feeding mechanism. Accordingly, when a large number of sheets are stacked, a height of a sheet bundle is increased, with the result that the sheet feeding device itself is increased in size. Further, in order to continuously supply the sheets to the separating/feeding mechanism, it is necessary that a sheet placing table be gradually and vertically raised so that an upper surface of the sheet bundle is held in contact with the separating/feeding mechanism. Therefore, increase in sheet stacking amount leads to increase in size of a mechanism of the sheet placing table to be operated. In view of this, in Japanese Patent No. 2625024, there is proposed a sheet feeding device (so-called tandem sheet feeding device) capable of achieving reduction in load on the sheet placing table and reduction in height of the sheet feeding device in such a manner that a supplemental sheet placing portion is arranged next to the sheet placing table. When sheets on the sheet placing table are used up, sheets on the supplemental sheet placing portion are automatically moved onto the sheet placing table, thereby being capable of feeding the sheets continuously without interrupting image forming operation.
In Japanese Patent No. 2625024, a sheet sensor configured to detect existence or nonexistence of the sheets is arranged on each of the sheet placing table and the supplemental sheet placing portion. When the sheet sensor on the sheet placing table detects “out of sheet”, a message of “out of sheet” is displayed on a display portion. When the sheet sensor on the supplemental sheet placing portion detects “out of sheet”, a message of “supply sheet” is displayed on the display portion. In consideration of a main body size and the like, a sheet size capable of being placed on the tandem sheet feeding device including the supplemental sheet placing portion is limited to a predetermined size (A4 size or letter size).
However, when the sheets are moved from the supplemental sheet placing portion to the sheet placing table after the user mistakenly places sheets having a size (for example, a B5 size) smaller than the predetermined size on the supplemental sheet placing portion, the sheet sensor on the sheet placing table cannot detect “existence of sheet”. Accordingly, the message of “out of sheet” is displayed on the display portion. In this case, when the predetermined-size sheets are placed on the sheet feeding device, although the sheet feeding device can be used normally, the user may misunderstand that a malfunction occurs in the sheet feeding device, and stop use of the sheet feeding device. Meanwhile, also when the sheets are not normally moved from the supplemental sheet placing portion to the sheet placing table though the predetermined-size sheets are properly placed on the supplemental sheet placing portion, the user cannot specify a cause of an abnormality. Accordingly, even under a situation in which the sheet feeding device breaks, the user may repeatedly place the predetermined-size sheets.